Role of microtubules and myosins in Fc gamma receptor-mediated phagocytosis.

Microtubules serve as tracks for vesicular traffic in both phagosome formation and phagosome maturation. In the process of phagosome formation, endomembrane vesicles are delivered to the membrane of the forming phagosomes to supply membrane. This localized endomembrane delivery, referred to as focal exocytosis, facilitates pseudopod extension for the purpose of engulfing large particles. The microtubule-based transport system is the most likely candidate for such targeted vesicle trafficking to the forming phagosomes. During their maturation process, phagosomes interact with early and late endosomes and finally fuse with lysosomes. Although phagosomal membrane fusion with other membranous compartments does not require microtubules, bi-directional transport and positioning of the two organelles on microtubules are necessary for their close positioning and subsequent membrane fusion. Microtubules are also responsible for vesicle trafficking along the antigen presentation pathway for phagocytosed materials. Some classes of myosin are involved in diverse processes of Fc gamma receptor (FcgR)-mediated phagocytosis as force generators and actin-based transport motors. The role of myosin II in phagocytic cup squeezing is complementary to the classical zipper closure model. Myosin Ic and myosin X seem to be key players in extending and closing phagocytic-cup pseudopod. Other classes of myosin may also be involved in phagosomal movement. Myosin V may control short-range phagosome movement and relay phagosomes to the long-range linear transport system using microtubules.